The present invention relates to an optical transmission apparatus used in a wavelength division multiplexed (WDM) optical transmission system. In particular, the invention relates to a WDM optical transmission apparatus in which there is performed a drive control that avoids an occurrence of inter-channel crosstalk caused by wavelength variations during transient states when an apparatus is started and stopped.
In a WDM optical transmission system, wavelengths of a plurality of optical signals to be wavelength multiplexed are standardized by the ITU (International Telecommunication Union). Most conventional optical transmission apparatuses used in WDM optical transmission systems are provided with a function that performs an automatic wavelength (frequency) control (hereunder abbreviated to AFC) in order to stabilize the wavelength of output light. This AFC function is a function that, for example, detects a wavelength of output light from a laser diode (LD), feeds back the detection result to control the temperature of the LD in order to maintain the wavelength of output light to be constant, which is realized by a so-called wavelength locker or the like.
FIG. 1 and FIG. 2 are block diagrams showing structural examples of conventional WDM optical transmission apparatus.
In the structural example of FIG. 1, light transmitted from an LD 100 is modulated by an external modulator 110, and its optical signal is output to the exterior via a wavelength locker 120 and an optical filter 150. In the wavelength locker 120, a wavelength of the optical signal from the external modulator 110 is detected by a wavelength detecting section 121 and the detection result is transmitted to a wavelength control circuit 122. In the wavelength control circuit 122, a signal for controlling the temperature of the LD 100 is generated such that the wavelength of the output light reaches a required target wavelength to be transmitted to a temperature control circuit (ATC) 130. In the temperature control circuit 130, the operating temperature of the LD 100 is controlled according to the signal from the wavelength control circuit 122, and a temperature monitor signal. Furthermore, a drive current to the LD 100 is controlled by a drive current control circuit 140. As a result, during normal operation, an optical signal is transmitted in a state where its channel interval is constant (for example 0.8 nm). Moreover, by installing an optical filter 150 such as a bandpass filter (BPF) on the output side of the wavelength locker 120, an occurrence of inter-channel crosstalk caused by wavelength variations at starting and during stopping of the apparatus is suppressed by hardware.
Furthermore, in the structural example of FIG. 2, the wavelength locker 120 monitors light emitted from the rear face of the LD 100 and performs a wavelength control. Also by installing an optical switch 160 on the output side of the external modulator 110, the occurrence of inter-channel crosstalk caused by wavelength variations is suppressed. Here, a modulator may be used as the optical switch 160.
In a conventional WDM optical transmission apparatus as described above, in order to avoid the occurrence of inter-channel crosstalk, an optical filter, an optical switch or the like is provided. However, since an optical filter, optical switch or the like needs to be provided for each wavelength light, the overall construction of the transmission apparatus becomes complicated, hence causing a problem in that it is difficult to achieve miniaturization and low cost. Furthermore, in a case where an optical filter is used, there is a disadvantage in that a large number of optical filters having different characteristics must be prepared. Moreover, there is also a problem in that by inserting optical filters, optical switches or the like, losses become large.
The present invention takes the above situation into consideration with the object of providing a WDM optical transmission apparatus capable of suppressing an occurrence of inter-channel crosstalk, by controlling the temperature of a light source and its drive current appropriately without optical filters, optical switches or the like.
In order to achieve the above object, a WDM optical transmission apparatus of the present invention used in a WDM optical transmission system for transmitting a wavelength division multiplexed signal light including a plurality of different optical signals, comprises: a light source for generating light whose wavelength is changed according to the temperature and a drive current; temperature control means for controlling the temperature of the light source so that a wavelength at the starting of emission of the light source is stabilized in an allowable range of optical output wavelength set in advance, based on wavelength characteristics for the light source drive current and intervals between adjacent optical signal wavelengths; drive current control means for controlling the drive current applied to the light source according to the allowable range of the optical output wavelength; wavelength control means for detecting a wavelength of light output from the light source and controlling the temperature of the light source based on the detection result, to lead the optical output wavelength into the vicinity of a preset target wavelength; and operation control means for controlling the start and stop of respective control operations of the temperature control means, the drive current control means and the wavelength control means at predetermined timing respectively corresponding to the time of optical output generation and the time of optical output stop.
According to such a construction, during transient states when the apparatus is started and stopped, variations occur in optical output wavelength due to changes of temperature and drive current of the light source. However, the temperature and drive current of the light source are controlled by the temperature control means, the drive current control means and the wavelength control means such that variations of this optical output wavelength are held within the allowable range set in advance. At this time, by controlling the start and suspension of the respective control operations of the temperature control means, the drive current control means and the wavelength control means, by the operation control means at the predetermined timing, it becomes possible to avoid an occurrence of inter-channel crosstalk without installing optical filters, optical switches or the like.
For a specific construction of the abovementioned WDM optical transmission apparatus, the arrangement is preferably such that the operation control means starts the control operation of the temperature control means when the optical output is generated, starts the control operation of the drive current control means when the temperature control is stabilized, starts the control operation of the wavelength control means when the drive current control is stabilized, and furthermore, the operation control means stops the control operation of the wavelength control means when the optical output is stopped, stops the control operation of the drive current control means when the temperature of the light source is stabilized by the control operation of the temperature control means, and stops the control operation of the temperature control means when there is no supply of drive current to the light source and emission is stopped.
Furthermore, the drive current control means may be constituted to perform an automatic current control for controlling the drive current supplied to the light source to be constant, or perform an automatic power control for controlling the drive current such that the optical output power from the light source is kept constant. Moreover, in a case where automatic power control is performed, by providing a circuit construction using a differential amplification method, maximum drive current is restricted, and hence it is possible to prevent an over current supply to the light source.
Furthermore, for a specific construction of the temperature control means, the arrangement may also be such that a reference temperature being a target of the control operation, is given according to a reference voltage set inside the apparatus, or is set according to a reference voltage applied from outside the apparatus.
Moreover, for an improvement to realize high output of the abovementioned WDM optical transmission apparatus, the arrangement may be such that the drive current control means is capable of setting in stepwise a reference value being a target of the control operation according to a plurality of reference voltages, and the operation control means starts the control operation of the temperature control means when the optical output is generated, starts the control operation of the drive current control means when the temperature control is stabilized, starts the control operation of the wavelength control means when the drive current control is stabilized, and when the wavelength control is stabilized, switches the reference voltage of the drive current control means to a higher level by one step each time, and continuously repeats the switching of the reference voltage each time the wavelength control is stabilized.
Alternatively, for another improvement to realize high output of the abovementioned WDM optical transmission apparatus, the arrangement may also be such that the operation control means starts the control operation of the temperature control means when the optical output is generated, starts the control operation of the drive current control means when the temperature control is stabilized, and thereafter, during a period until the drive current reaches a target reference value, repeats a series of operations in which, when the optical output wavelength detected by the wavelength control means reaches an upper limit wavelength set within the allowable range, the control operation of the wavelength control means is started and also the control operation of the drive current control means is suspended, and when the optical output wavelength is led into the vicinity of the target wavelength by the control operation of the wavelength control means, the control operation of the drive current control means is resumed.
According to the construction applied with such an improvement, it becomes possible to increase the drive current to the light source while realizing the control within the allowable range of the optical output wavelength, to achieve a high output of the present apparatus.
In addition, for a specific construction of the operation control means, the arrangement may be such that there is provided a timer for performing timing operations in synchronization with an alarm signal input for instructing the generation and stop of optical output, and outputting a signal for controlling the start and stop of the respective control operation of the temperature control means, the drive current control means and the wavelength control means according to predetermined timing set in advance.
According to such a construction, the start and stop of the respective control operations of the temperature control means, the drive current control means and the wavelength control means are controlled according to timing based on timing operations of the timer.
Alternatively, for another specific construction of the operation control means, the arrangement may be such that there is provided a temperature control alarm generation section for generating a temperature control alarm signal indicating the control state of the temperature control means, a drive current control alarm generation section for generating a drive current control alarm signal indicating the control state of the drive current control means, and a sequencing section for controlling the start and stop of the respective control operations of the temperature control means, the drive current control means and the wavelength control means based on external alarm signals for instructing the generation and stop of optical output, the temperature control alarm signal and the drive current control alarm signal.
According to such a construction, operation timing of each section is controlled according to actual control states based on the respective alarm signals generated inside the apparatus.